Control circuit for gas discharge tubes



Aug. 10, 1948. J. H. ENGLAND v 2,446,533

CONTROL CIRCUIT FOR GAS DISCHARGE TUBES Filed April 4, 1946 H/GH- v 6 ClaOVLZAbGE 2 mm/ 06.

T L n RECTIFIER s 3 SOURCE //WENTOR Joy/v A. ENGLAND Patented Aug. 10, 1948 John H. England, Wellesley Hills, Mass., assignor to Raytheon Manufacturing Company, Newton, Mass, a corporation of Delaware Application April 4, 1946, Serial No. 659,458

7 Claims.

This invention relates to electrical circuits, and more particularly to a discharge starting circuit for gas discharge tubes.

An object-of this invention is to devise a control circuit for gas discharge tubes in which a sufliciently low negative potential is first applied to the control element of the tube to start the discharge, but in which the control element is thereafter returned rapidly and, automatically to its original high negative potential.

A further object is to devise a discharge starting circuit for gas discharge tubes, operated by a relay, in which the discharge, once it is started in response to energization of the relay, will be put out and will not be restarted even though the relay be maintained energized.

The foregoing and other objects of the invention will be best understood from the following description of an exem-plification thereof, reference being had to the accompanying drawing, wherein the single figure is a diagrammatic representation of one means for carrying out the in vention.

In a certain type of welding equipment, a con denser, charged from a high-voltage rectifier source, is discharged through a gas discharge tube, for example a thyratron, in response to closure of a control switch by the operator, to supply welding current to a pair of electrodes. The closure of the switch so controls therectifier source as to disconnect it from the condenser during the welding operation, but it is automatically reconnected to the condenser after a certain time has elapsed, in order to start charging the condenser for the next welding operation. It is essential that the recharging of the condenser begin as soon as possible after the welding operation has been completed, in order that it may be charged before the next welding cycle,

in time, is begun. In the systems of the prior art, if the operator accidentally or improperly held the control switch closed iortoo long a period, the thyratron voltage remained such that the thyratron would be re-fired at the instant when the rectifier was reconnected to the condenser, and, instead of the source charging the condenser, said source would be short-circui-ted through the anode-cathode path of the thyratron. In the system of the present invention, the potential of the grid of the thyratron is firstreduced to a relatively low value, with respect to its cathode, to fire the tube and discharge the condenser, in response to closure of the control switch by the operator. However, the grid poe tential is then rapidly and automatically returned to its high initial value, and this is accomplished even if the control switch remains closed. Because of the rapid recovery of the grid voltage to its initial (high negative) value, the thyratron will not be re-fired when the rectifier is re-connected to the condenser, even if the control switch remains closed, so that the source may recharge the condenser, .as desired, for the next welding cycle.

Referring, now, to the drawing, a high-voltage controlled rectifier I supplied from 'a source of alternating current 29 has positive and negative output terminals 2 and 3, respectively, and control terminals 4 and5. Rectifier l is so arranged that, when acircui-t is closed between control terminals 4 and '5, said rectifier will be eifectively disconnected or isolated from its output terminals 2' and 3 for a predetermined period. after which it will be automatically reconnected to said out put terminals. Power condenser 6 is connected across output terminals 2 and 3, so as to be charged from said rectifier. A gaseous discharge device I, for example a thyratron, has anode 8, grid or control element 9, and cathode l ll. Anode 8 is connected to output terminal 2, while cathode lll is .connected through the primary l5 of a weldingtransformer P6 to output terminal 3,

.cathodepa'th of discharge device I and primary 15 in series. Welding electrodes H! are connected across the secondary ll of transformer l6.

Grid 9 of tube 1 is connected, through a resister H, to the negative output terminal l2 of a direct current source l3, the positive output ter minal l4 of which is connected to cathode 10. Source I 3 may have an output voltage of 125 volts, for example, while resistor ll may have a resistance of 100,000 ohms, for example. Relay l9 has an armature 20, a pair of normally-open contacts 21, a second pair of normally-opencontacts 22, a pair of normally-closed contacts 23, and an operating winding 24. Winding 24 is con nected, in series with a control switch 25, across the terminals 12 and M of source l3, so as to be energized from said source when switch 25 is closed. One of contacts 2| is connected, by means of lead 21, to control terminal 4 of rectifier I, while the other of contacts 2| is connected, by means of lead 28, to control terminal 5 of said rectifier.

,Condenser 26 has one terminal thereof conf nected to cathode Ill, while its other terminal is connected, in series with relay contacts 22, to grid 9, between said grid and resistor ll. Contacts 23 of the relay are connected in shunt with condenser 26. Condenser 26 may have a capacitance of 0.005 microfarad, for example.

Originally, condenser 6 is charged, being charged from rectifier I. Condenser 26 is originally discharged, as a result of normally -closed relay contacts 23 shunted thereacnoss'. With the polarities shown, tube I is rendered non-conducting because its grid 9 is highly negative with respect to its cathode, having the high negative potential of source I3 applied thereto; a

When control switch 25 is closed to start the operation, winding 24 of relay I9 is connected across source I3, so that relay I9 is energized to close contacts H and 22 and open contacts 23. Relay I9 remains energized as long as switch 25 is closed. Closure of contacts 22 connects con denser 26 in series with resistor I I across source 7 I3, while opening of contacts 23 opens the shunt circuit across said condenser. Condenser- 26 starts to charge from source I3 through resistor 11. The initial inrush of current into the condenser is drawn. through resistor II, causing most of the bias voltage of source I3 to appear as a voltage drop across resistor I I, and the remainder, which is low enough to allow tube 1 to fire, to appear across the condenser 26, which is connected directly between cathode I0 and grid 8. Tube I therefore fires, allowing condenser 6 to discharge therethrough and through the primary I5 of the welding transformer. During this time interval, due tothe closure of relay contacts 21 completing a circuit between control terminals 4 and 5 of controlled rectifier I, said rectifier is disassociated from its output terminals 2 and 3. As condenser 6 discharges, the voltage between anode 8 and cathode III will be reduced, until a point is reached at which the said voltage isinsufiicient to maintain the arc in tube 1, so that the arc .goes out. Rectifier I is still disassociated from its output terminals at this time. When rectifier I is reconnected to condenser 6 to recharge it, thus placing a positive potential on anode 8, the arc in tube I will not be re-established provided that a sufiiciently negative potential exists on .grid 9 at this time.

As condenser 26 becomes charged the voltage across resistor Il decreases, due to, decrease in thecurrent flowing therethrough, and the voltage across the condenser increases, due to its becoming charged. When the condenser, 26 has fully charged, the grid to cathode voltage of tube I hasreturned to its original high negative value. The time required to charge condenser 26 is dependent upon the capacitance of said condenser and the resistance of resistor II, and is made lessthan the time elapsing between isolation or rectifier I from its output terminals and reconnection of said rectifier thereto. Due to the high negative potential on grid 9 at the instant when rectifier I is reconnected to output terminals 2-3, as a result of the charge on condenser 26, the arc in tube I will not be re-fircd at said instant. Condensec26 remains charged as long .as contacts 23 remain open due to the energization of relay I9, thereby maintaining a high negative bias on grid 9 as long as relay I9 is energized. Therefore, tube .1 will not refire even though relay I6 is kept energized due to closure of switch 25 and even though theanodecathode voltage of tube 1 has risen above the value necessary to maintain, the arc, provided of course that the time constantof condenser .26 is such that it has .fully charged before the anode-cathode voltage of tube I again rises above the value necessary to maintain the arc, as stated above. of course, if relay I9 is deenergized, opening of contacts 22 entirely disconnects condenser 26 from the grid 9, so that the high negative bias provided by source I3 is maintained on said grid to preventfiring of the tube.

-Open ing",of switch "25 deener'gizes relay I9, opening contacts 2| and 22, and closing contacts v23. Closure of contacts 23 discharges condenser 26, while opening of contacts 22 disconnects said condenser from source l3. Opening of contacts 2| readies the control circuit of rectifier l for the next operation. Tube 1 can then be re- ;fired whenrelay I9 is re-energized for the next operation.

Of course, it is to be understood that this invention is not limited to the particular details as described above, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims 'be given a broad interpretation commensurate with the scope of this invention within the art.

What is'claimed is:

1. A discharge-starting circuit for gaseous discharge devices, comprising a gaseous discharge device having at least anode, cathode, and control elements, a source of direct current, means connecting the negative side of said source through a resistor to .said control element, means connecting the positive side of said source to said cathode, condenser having one terminal connected to said cathode, a relay having a pair of normally-open contacts and means connecting one of said contacts to said condenser and the other of said contacts to said control element.

2. Adischarge-starting circuit for gaseous discharge devices, comprising a gaseous discharge device having at least anode, cathode, and control elements, .a source of direct current, means connecting the negative side of said source through a resistor to said control element, means connecting the positiveside of said source to said cathode, a condenser having one terminal connected to said cathode, a relay having, a pair of normallyopen contacts and a pair of normally-closed contacts,. means connecting said normally-closed contactsacross said condenser, and means connecting one of; said normally-open contacts to said condenser and the other of said normally-open contacts to said control element.

3. A discharge-starting circuit for gaseous discharge devices, comprising a gaseous discharge device having at least anode, cathode, and grid elements, a source of direct current,.means connectin the negative side of said source through aresistor to said grid, means connecting the positiveside of said source to said cathode, a condenser having one terminal connected to said cathode, a relay having a control winding, 3. pair of normally-open contacts and a pair of normally-closed contacts, means connecting said normally-closed contacts across said condenser, means connecting one of said normally-open contacts to said condenser and the other of said normally-open contacts to said grid, and means for energizing said control winding to close said normally-open contacts and open said normallyclosed contacts.

4. .A discharge-starting circuit for gaseous discharge devices, comprising a gaseous discharge device having at least anode, cathode, and control elements, a source of direct current, means connecting the negative side of said source through an impedance to said control element,

means connecting the positive side of said source to said cathode, a normally-discharged condenser having one terminal connected to said cathode, and means operable at will to intermittently connect the other terminal of said condenser to said control element and thereby also to said source, through said impedance, to charge said condenser therefrom.

5. A discharge-starting circuit for gaseous discharge devices, comprising a gaseous discharge device having at least anode, cathode, and control elements, a source of direct current, means connecting the negative side of said source through a resistor to said control element, means connecting the positive side of said source to said cathode, a normally-discharged condenser having one terminal connected to said cathode, and means operable at will to intermittently connect the other terminal of said condenser to said con.- trol element and thereby also to said source, through said resistor, to charge said condenser therefrom.

6. A discharge-starting circuit for gaseous discharge devices, comprising a gaseous discharge device having at least anode, cathode, and control elements, a source of direct current normally rendering said device non-conducting by biasing said control element negatively with respect to said cathode, a normally-discharged condenser having one terminal connected to said cathode, and means operable at will to intermittently connect the other terminal of said condenser to said control element and thereby also to said source, through a resistor, to charge said condenser, the

6 flow of charging current through said resistor providing a voltage drop which reduces said negative bias sufiiciently to allow said device to conduct.

7. A discharge-starting circuit for gaseous discharge devices, comprising a gaseous discharge device having at least anode, cathode, and control elements, a source of direct current normally rendering said device non-conducting by biasing said control element negatively with respect to said cathode, a, normally-discharged condenser having one terminal connected to said cathode, and means operable at will to intermittently connect the other terminal of said condenser to said control element and thereby also to said source, through a resistor, to charge said condenser, the flow of charging current through said resistor providing a voltage drop which reduces said negative bias sufficiently to allow said device to conduct, said condenser thereafter becoming charged with a polarity such as to place a negative potential on said control element.

JOHN H. ENGLAND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,114,614 Schlesinger Apr. 18, 1938 2,169,023 Dawson Aug. 8, 1939 2,320,916 Dawson June 1, 1943 2,363,305 Faulk Nov. 21, 1944 

